Radiation imaging systems typically are used to generate images of the distribution of radiation either transmitted through an object or emitted from an object. Such radiation is not visible to the naked eye. The various modalities of imaging distributions of radiation include Transmission Imaging and Emission Imaging. Both of these modalities are applied in medicine.
Classical transmission imaging or x-ray radiography is a technique wherein the radiation is generated externally and caused to propagate through an organ or body to the detector. In this way an image of the distribution of radiation absorption, or transmission, in the organ or body is obtained. One of the examples of the transmission imaging is mammography used for providing images of the breast in sufficient detail to assure high sensitivity screening for abnormal tissue.
Mammography is accepted as the best means of screening for non-palpable breast cancer. However, signatures of breast cancer, such as micro-calcifications or masses, seen for most malignant lesions, are also associated with benign processes. Thus, while the sensitivity of mammography is reported to be about 85%, its specificity is only 20-30%, and only about 30% of biopsies based on mammography are positive according to the following papers: xe2x80x9cTc-99m-Sesta MIBI Prone Imaging in Patients (PTS) with Suspicion of Breast Cancer (Ca)xe2x80x9d by Khalkhali, I., I. Mena, E. Jouanne, L. Diggles, K. Alle, S. Klein in J. Nucl. Med., 24:140P, May 1993, xe2x80x9cSensitivity and specificity of first screen mammography in the Canadian National Breast Screening Study: a preliminary report from five centresxe2x80x9d by Baines C J; Miller A B; Wall C; McFarlane D V; Simor I S; Jong R; Shapiro B J; Audet L; Petitclerc M; Ouimet-Oliva D; et al; in Radiology, 160:295-298, (1986), and xe2x80x9cMammographic parenchymal patterns: risk indicator for breast cancer?xe2x80x9d by Tabar, L. and Dean P B, in JAMA 247:185-189, (1982).
Excisional biopsies on a false positive patient result in large unnecessary costs and the scarring that can cause difficulties in interpretation of future mammograms according to a paper titled xe2x80x9cRadiographic Breast Anatomy: Radiological Signs of Breast Cancerxe2x80x9d by Shaw de Paredes E. in Syllabus: A Categorical Course in Physics and Technical Aspects of Breast Imaging, eds. A. G. Haus and M. J. Yaffe, RSNA Publications, Oak Brook Ill., 1992. Many centers now use stereotactic systems for core biopsies immediately after mammography, while the breast is compressed in the same position as in the mammogram. While the stereotactic procedure is somewhat less traumatic, the cost is still significant, especially for the 70% of patients who had false positives.
In emission imaging (xe2x80x9cNuclear Medicinexe2x80x9d) radiation is generated within the organ by radiopharmaceutical or other radiation bearing substance which passes through or in some cases is designed to accumulate in the organ. Many emission imaging applications exist including single photon planar imaging and Single Photon Emission Computed Tomography (SPECT) for imaging the structure or function of internal organs.
Gamma-ray cameras employed in single photon emission imaging applications typically consist of a collimator for xe2x80x9cfocusingxe2x80x9d the gamma-rays, a detector for determining the position of each incident gamma-ray and a device for displaying the acquired images. Traditional gamma-ray cameras utilize scintillation detectors coupled to photomultiplier tubes (PMT""s) for detecting the light emitted from the scintillator. This development is described in a paper titled xe2x80x9cScintillation Cameraxe2x80x9d, by Hal O. Anger, published 1958, The Review of Scientific Instruments, Vol. 29 No. 1 and in a paper titled xe2x80x9cGamma-Camera Systems,xe2x80x9d by M. D. Short, in 1984, Nuclear Instruments and Methods, Vol. 221. In these cameras, the scintillator is generally a single crystal (70 cm diameter) which is coupled to multiple PMT""s. Each PMT covers several square centimeters of area of the scintillation crystal. Recently, smaller, higher spatial and energy resolution gamma-ray cameras dedicated to particular applications have been developed or are under development. These new cameras are based on PMT""s, position sensitive PMT""s (PSPMT) or solid state detectors. The solid state detector based camera can be one which has a scintillator coupled to a solid state detector. In this case the solid state detector has replaced the PMT or PSPMT as the device which converts the light emanating from the scintillator into electrical signals. A typical example of such an implementation is a gamma-ray camera based on a silicon pin photodetector array coupled to CsI(Tl) scintillator described in U.S. Pat. No. 5,773,829, which is incorporated by reference in its entirety into the present disclosure. Another approach utilizes a solid state detector, which directly converts the radiation to electrical signals.
An example of emission imaging is breast imaging using the radiopharmaceutical MiraLuma(trademark) (Tc-99m-Sestamibi). Recent developments in testing of this radiopharmaceutical, which was initially developed for measuring blood flow in the myocardium, show that the compound is also selectively taken up in tumors, apparently in proportion to the malignancy of the tumor. The compound compares favorably with Tl-201 in tumor uptake as described in the papers titled xe2x80x9cIn vitro uptake of technetium-99m-teboroxime in carcinoma cell lines and normal cells: comparison with technetium-99m-Sestamibi and thallium-201 xe2x80x9d by Maublant J C; Zhang Z; Rapp M; Ollier M; Michelot J; Veyre in A. J. Nuc. Med., 1993 November, 34 (11):1949‥52, xe2x80x9cThallium-201 versus technetium-99m-MIBI SPECT in evaluation of childhood brain tumors: a within-subject comparisonxe2x80x9d by O""Tuama L A; Treves S T; Larar J N; Packard A B; Kwan A J; Barnes P D; Scott R M; Black P M; Madsen J R; Goumnerova L C et al. in J. Nuc. Med., 1993 July, 34(7):1045-51., and xe2x80x9cConcordant uptake of Tc-99m Sestamibi and Tl-201 in unsuspected breast tumorxe2x80x9d by Campeau R J; Kronemer K A; Sutherland C M, in Clin. Nucl. Med., 1992 December, 17 (12):936-7. It is believed that the Tl-201 uptake is a measure of blood flow, while the Sestamibi is sensitive to tumor metabolic rate or malignancy. In addition, Sestamibi""s mechanism of uptake fixes the compound and minimizes redistribution. Uptake of Sestamibi is also very rapid. It is fixed in the heart, liver and tumor in about 10 minutes, and has a maximum uptake in the tumor at about 5 minutes. Recent reports such as the one reported in papers on detection of breast tumors using Sestamibi titled xe2x80x9cScintimammography: the complementary role of Tc-99 m Sestamibi prone breast imaging for the diagnosis of breast carcinomaxe2x80x9d by I. Khalkhali, J. A. Cutrone, I. G. Mena, L. E. Dingles, et al., in Radiology 196 (1995):421-426, and xe2x80x9cTechnetium-99m-Sestamibi Prone Scinti-mammography to Detect Primary Breast Cancer and Axillary Lymph Node Involvementxe2x80x9d by Taillefer, R., Robidoux, A., Lambert, R., Turpin, S., and Laperriere, J. in J. Nuc. Med. 36:1758, October 1995, all give sensitivities and specificities in the neighborhood of 90%. Recently, equally encouraging results were also reported for Tc-99m-Methylene Diphosphonate (MDP) with a sensitivity of 92% and a specificity of 95% in a paper titled xe2x80x9cTechnetium-99m-Methylene Diphosphonate Scintimammography to Image Primary Breast Cancerxe2x80x9d by Piccolo, S., Lastoria, S., Mainolfi, C., Muto, P., Bazzicalupo, L., Salvatore, M. in J. Nuc. Med. 1995. 36:718-724.
Part of the 10% or so of the lesions missed in the studies such as the ones reported by Kalkhali and Taillefer cited above were due to the small size and/or lower uptake of the particular lesions. In one study reported in a paper titled xe2x80x9cTechnetium-99m-sestamibi uptake in breast tumor and associated lymph nodesxe2x80x9d by J. Maublant, M. de Latour, D. Mestas, et al. in J. Nucl. Med. 37 (1996):922-925, patients were injected with Tc-99 m Sestamibi and imaged with a scintillation camera one day prior to a second injection of Sestamibi prior to excisional breast and/or axillary biopsy. All patients had positive mammograms, and 78% had positive scintimammograms. It was found that all excised tumor tissue had significant Tc-99 m Sestamibi uptake (6.13xc2x12.37 tumor to tissue ratio). This included tumors missed with scintimammography. The implication is that the uptake of Tc-99 m Sestamibi is extracted into essentially all tumors and that the false negatives with Tc-99 m Sestamibi scintimammography are due to the limitations in the sensitivity, resolution and clinical placement during the procedure of current scintillation cameras. Thus, it would be desirable to provide a scintillation camera with the necessary sensitivity, resolution and clinical placement to prevent false negatives.
In emission imaging of the breast using MiraLuma(trademark) (Tc99m-Sestamibi), 10-20 mCi of Tc-99m-Sestamibi is a typical dose as reported in papers such as the ones by Kalkhali et al. and Taillefer et al. referenced above. The resulting whole body dose is 0.3 Rad (3mGy), according to the above referenced Kalkhali paper, with minimal dose to the breast. This whole body dose is less than the dose from a standard chest X-ray and comparable to the dose from a typical mammogram. For example, a typical mammogram with a measured entrance exposure of 1 Roentgen from a Mo/Mo target/filter system at 30 kVp with a 0.36 mm aluminum HVL yields a glandular dose of 0.19 Rad (1.9mGy) as described in the Mammography Quality Control Manual, 2nd Ed., eds. R. E. Hendrick, L. Bassett, M. A. Botsco, et al., American College of Radiology 1994:159-165.
The standard scintillation camera of the prior art is too bulky to place in a position close to the breast and still image without the bulk of the body as background. Thus, the radiopharmaceutical which is fixed in the heart, liver, and other organs contributes a significant background and scattered radiation component. The background and scatter degrade the image quality. Thus, it would be desirable to provide a scintillation camera that could be placed close to the breast image without unwanted background.
In U.S. Pat. No. 5,519,221, which is incorporated by reference in its entirety into the present disclosure, the gamma-ray camera head is shown to be separated by substantial distance from the compressed breast, and additional materials such as a compression plate are placed between the breast and the gamma-ray camera head. In this reference, the gamma-ray camera is specifically separated from the immobilization apparatus in order to facilitate easy movement of the gamma-ray camera head with respect to the immobilized organ for the purpose of obtaining multiple projections. With such an apparatus, it is not possible to achieve the benefits of extreme proximity to the lesion.
For stereotactic biopsy, partial compression (10 lbs/in2) is used. This typically provides compression to approximately 5 cm, which is tolerated for 30-45 minutes as described in xe2x80x9cChapter 7: Stereotactic large-core breast biopsyxe2x80x9d, in Percutaneous Breast Biopsy, eds. S. H. Parker, W. E. Jobe, Raven Press, Ltd., New York, 1993 by S. H. Parker. Typical full compression for standard x-ray mammography, at 18 lbs/in2 is usually tolerated for only 1 minute, compressing the breast to approximately 4 cm. An example of the caudal compression is that achieved with the Instrumentarium(trademark) mammography unit, which allows extended compression periods with little pain or trauma and still provides approximately 80% of full compression. The thickness of the partially compressed breast is approximately 2-6 cm, which is excellent for imaging with a scintillation camera. In typical use with the small gamma-ray camera one injects the patient with Tc-99m-Sestamibi and images a suspicious lesion within 10-20 minutes of the initial diagnosis.
The integral gamma-ray camera and compression member of the present invention eliminates many of the disadvantages of the prior art apparatus for examining the breast for suspicious lesions. A small gamma-ray camera is attached to a mammography unit or to a stand-alone system in such a way that the gamma-ray camera is in direct contact with the breast as part of the breast compression system. Incorporating the gamma-ray camera into the breast compression fixture assures minimum distance between the lesion in the compressed breast tissue and the gamma-ray camera. All unnecessary materials are removed from between the compressed breast and the gamma-ray camera, with the possible exception of a pad or sheet of suitable material for cushioning the breast and minimizing patient discomfort. The gamma-ray camera comprises a collimator and a gamma-ray sensitive imaging detector. The gamma-ray camera is at once a part of the means for breast compression as well as a gamma-ray imaging system for providing an image of radio tracer distribution in the breast. An example of a suitable radio tracer is the radiopharmaceutical MiraLuma(trademark) (Tc-99m-Sestamibi).
The approach of the present invention allows for the closest distance to the lesion under examination and minimizes radiation scatter from breast tissue itself and other objects placed between the gamma-ray camera and breast. The gamma-ray camera apparatus has resolutions and signal to noise ratios that are significantly better (up to factor of 2 for resolution and greater than 60% for signal to noise) than standard scintillation cameras. This improvement in signal to noise is due to higher sensitivity and better spatial resolution achieved by the close proximity of the camera to breast lesions. In order to obtain these improvements it is important to apply the compression with the gamma-ray camera itself being part of the compression mechanism and to eliminate any additional media placed between the imaging gamma-ray camera and breast.
The proximity affords a transition from collimator limited camera resolution, which is characteristic of Anger camera imaging intended to image objects at depth, to a regime of intrinsic detector-limited resolution of relatively shallow objects which are fairly close to the detector. Thus the present invention leads to a significant improvement in spatial resolution compared to what can be obtained with the apparatus described in U.S. Pat. No. 5,519,221.
The small size of the camera and substantial improvement in geometric efficiency afforded by the proximity to the object being imaged allows the camera to be positioned for breast imaging without the remainder of the body in the background. This leads to improvements in image quality due to reduction of background and scattering due to parts of the body other than the breast.
The apparatus allows for alignment of the breast and the gamma-ray camera head in a fixed position during each imaging session. This represents an improvement over the apparatus described in U.S. Pat. No. 5,519,221 where the organ (breast) is immobilized and the gamma-ray camera head is allowed to move with respect to the immobilized organ for the purpose of obtaining multiple projections. The apparatus of the present invention allows for positioning of the breast in different manners between the imaging sessions to obtain multiple projection views of the breast and to view the lesion with the least separation from the detector. Typical views of breast taken with the apparatus are similar to those used in mammographic projections.
Because scintimammography using the apparatus of the present invention takes approximately 10 minutes to perform, the use of partial compression and newer caudal compression techniques is appropriate, thus reducing patient discomfort as compared to the use of full compression.
Because of the increased efficiency and better signal to noise ratio afforded by the apparatus of the present invention, it is possible to use lower doses of the radiopharmaceutical, or shorter image acquisition times, with equal or better image quality than is possible without the use of the subject invention.
Other features and advantages of the present invention will be apparent from the following detailed description when read in conjunction with the accompanying drawings.